A semiconductor made of a wurtzite structure compound, such as aluminum nitride (AlN), has a piezoelectric property and a pyroelectric property. Since the wurtzite structure compound has such properties, the wurtzite structure compound is applicable to various devices, such as electronic devices, optical devices and high-pressure proof/high-temperature proof electronic devices.
In fact, the wurtzite structure compound has been applied as a material for various electronic components, such as high-frequency oscillators, filters for high frequency, various sensors, switches, ultrasonic oscillators, electromechanical transducers for audible bandwidth and light emitting elements.
Among the wurtzite structure compounds, AlN has especially been drawing researchers' attentions since AlN was synthesized for the first time in the middle of the 19th century. AlN is a material of a hexagonal wurtzite structure compound, and is one of group III-V compounds. The wurtzite structure compound is a compound formed mainly by covalent bond, however AlN is different from other group III-V compounds. That is, AlN is characterized by slightly having ionicity.
The following is four basic characteristics of the wurtzite structure compound.
(1) chemically stable at high temperatures
(2) high in corrosion resistance with respect to molten metal
(3) excellent in electric insulation
(4) high in thermal conductivity
Moreover, it is well-known that the wurtzite structure compound has the piezoelectric property and the pyroelectric property since the wurtzite structure compound has a crystal form having no symmetry center.
As a method for synthesizing a thin film or a monocrystal from the wurtzite structure compound, various methods have been studied, such as a reactive sputtering that is one of physical vapor depositions (PVD), a molecular beam epitaxy, a laser ablation, an ion plating, a chemical vapor deposition (CVD) using aluminum chloride and ammonia gas and metalorganic CVD using trimethyl aluminum and ammonia gas.
Most of these studies aim to realize the practical utilization of an electronic/optical functional thin film made of the wurtzite structure compound. Especially, since AlN is the highest in the propagation speed of a surface acoustic wave (SAW) among piezoelectric substances, applying AlN as a SAW device, typically a SAW filter of high-frequency region, and a bulk acoustic wave (BAW) is expected.
Incidentally, in order to fully obtain properties of electronic component materials using the wurtzite structure compound, it is necessary in most cases to control the crystallinity and crystalline orientation of the wurtzite structure compound. On this account, an epitaxial technique using a monocrystalline substrate and a technique for controlling the crystallinity and the crystalline orientation by adding impurities have been proposed.
For example, an AlN film formed by sputtering has a polycrystalline structure in many cases. However, in order to utilize such AlN film as the piezoelectric substance, the orientation of a crystal axis needs to be controlled in a film-forming process. In the case of AlN, a (0002) surface orientation is ideal. Many studies have been made to realize this ideal.
Moreover, piezoelectric ceramics formed on a substrate as a thin film is required to have a high Curie point of 1,000° C. or more and be able to deal with a high vibration frequency of thousands of MHz or more. In the case of practically forming the piezoelectric ceramics on a substrate as a thin film, it is possible to accordingly select the piezoelectric ceramics having such properties.
Specifically, the piezoelectric ceramics having such properties is exemplified by perovskite oxide, LiNbO3 oxide and a wurtzite compound, and the wurtzite compound is exemplified by AlN, ZnO, etc. Between ZnO (zinc oxide) and AlN (aluminum nitride), a thin film oriented in a c-axis direction is advantageous in that the thin film can be formed even if the substrate is a sintered body (see Document 1 (Japanese Unexamined Patent Application No. 122948/1998 (Tokukaihei 10-122948), published on May 15, 1998)).
However, in order to obtain an effective piezoelectric property, not only the crystalline orientations but also spontaneous polarization directions of respective crystal grains need to be aligned in a uniform direction. This is because crystal grains polarized in opposite directions decrease the overall piezoelectric effect.
Unlike ferroelectric substances, such as lead zirconium titanate (PZT), a polarization treatment cannot be carried out with respect to the wurtzite structure compound after the film formation. Therefore, studies of the polarization have not been made until now. In the case of the wurtzite structure compound, the control of the polarization directions and the film formation need to be carried out simultaneously.
The present invention was made in view of the above-described problems, and is to provide a thin film containing the wurtzite structure compound whose crystalline orientation is satisfactory and the spontaneous polarization directions are aligned in a uniform direction, and a method for manufacturing the thin film.
Incidentally, in the case of applying a wurtzite crystalline structure compound as the electronic component material, the wurtzite crystalline compound needs to be, in many cases, complexed with a functional material having conductivity and other function(s). Therefore, it is necessary to form a laminate having a multilayer structure in which a layer made of the wurtzite crystalline structure compound whose crystallinity and crystalline orientation are controlled is formed on a functional material layer made of such arbitrary functional material (for example, see Document 2 (Japanese Unexamined Patent Application No. 142607/1985 (Tokukaisho 60-142607), published on Jul. 27, 1985)).
However, in the case of forming the layer made of the wurtzite crystalline structure compound on the functional material layer that is a ground, there is a problem in that it is difficult to control the crystallinity and crystalline orientation of the wurtzite crystalline structure compound.
In order to solve this problem, Document 3 (Japanese Unexamined Patent Application No. 48820/1982 (Tokukaisho 57-48820), published on Mar. 20, 1982) discloses a surface acoustic wave element including, as shown in FIG. 6, (i) a substrate 15, (ii) a first wurtzite structure piezoelectric thin film 16 formed on the substrate 15, (iii) electrodes 17 each of which is formed on the first wurtzite structure piezoelectric thin film 16 by photo-etching to be a predetermined shape and is made of a material having a face-centered cubic structure and (iv) a second wurtzite structure piezoelectric thin film 18 which is also formed on the first wurtzite structure piezoelectric thin film 16 while containing the electrodes 17. In the surface acoustic wave element, the first wurtzite structure piezoelectric thin film 16 functions so as to mainly improve orientations of the electrodes 17 and hence an orientation of the wurtzite structure piezoelectric thin film 18.
In Document 3, as shown in FIG. 6, the first wurtzite structure piezoelectric thin film 16 is formed between the substrate 15 and the electrodes 17 in order to improve the orientation of the second wurtzite structure piezoelectric thin film 18 formed on the electrodes 17 that are island shaped. However, depending on the electronic component material, it may be necessary to use a laminate in which a second wurtzite crystalline structure compound layer is formed on a functional material layer, such as the electrode 17 which not partially but entirely covers a first wurtzite crystalline structure compound layer.
However, in the case of forming the second wurtzite crystalline compound layer on the functional material layer which not partially but entirely covers the first wurtzite crystalline compound layer, there are problems in that an internal stress of the functional material layer is large and the second wurtzite crystalline structure compound layer is easily peeled off.
Moreover, in the case in which the functional material layer is made of a metal having a body-centered cubic structure or a hexagonal close-packed lattice structure, the control of the crystallinity and crystalline orientation of the wurtzite crystalline layer formed on the functional material layer is more difficult than that in the case in which the functional material layer is made of a material having the face-centered cubic structure, and actually a method for controlling these has not been found so far. This is because, as compared with the functional material having the face-centered cubic structure, (i) the functional material having the body-centered cubic structure or the hexagonal close-packed lattice structure is largely different in lattice constant from the wurtzite crystal and (ii) the functional material having the body-centered cubic structure is low in crystallinity.
The present invention provides not only (i) a thin film containing a wurtzite structure compound whose polarization directions are aligned in a uniform direction and (ii) a method for manufacturing the thin film but also (iii) a laminate having a multilayer structure in which a layer made of the wurtzite crystalline structure compound whose crystallinity and crystalline orientation are controlled is formed on a functional material layer regardless of the crystalline structure of the functional material layer and (iv) a method for manufacturing the laminate.